Always treat [call] members as closures during tracing.

sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart

Index: sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart b/sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart
index b1ba510d50e0f7f1e3d443896b592548184b31fd..b343193ec587fcaa7874458f4af1e902ae9fc318 100644
--- a/sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart
+++ b/sdk/lib/_internal/compiler/implementation/inferrer/node_tracer.dart
@@ -333,6 +333,15 @@ abstract class TracerVisitor implements TypeInformationVisitor {
 
   bool isClosure(Element element) {
     if (!element.isFunction()) return false;
+    /// Creating an instance of a class that implements [Function] also
+    /// closurizes the corresponding [call] member. We do not currently
+    /// track these, thus the check for [isClosurized] on such a method will
+    /// return false. Instead we catch that case here for now.
+    // TODO(herhut): Handle creation of closures from instances of Function.
+    if (element.isInstanceMember() &&
+        element.name == Compiler.CALL_OPERATOR_NAME) {
+      return true;
+    }
     Element outermost = element.getOutermostEnclosingMemberOrTopLevel();
     return outermost.declaration != element.declaration;
   }